Long Term Evolution (LTE)
Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM (also referred to as Single-Carrier Frequency Division Multiple Access (SC-FDMA)) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the downlink.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds (ms), each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 2. For a normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 microseconds (μs).
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., the base station transmits control information in each subframe about which terminal's data is transmitted to and upon which resource blocks the data is transmitted in the current subframe. This control signaling is typically transmitted in the first 1, 2, 3, or 4 OFDM symbols in each subframe, and the number n=1, 2, 3, or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3. The reference symbols shown in FIG. 3 are the Cell Specific Reference symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization, and channel estimation for certain transmission modes.
Physical Downlink Control Channel (PDCCH) and Enhanced PDCCH (EPDCCH)
From LTE Release 11 (Rel-11) onwards, the above described resource assignments can also be scheduled on the Enhanced Physical Downlink Control Channel (EPDCCH). For Release 8 (Rel-8) to Release 10 (Rel-10), only Physical Downlink Control Channel (PDCCH) is available.
The PDCCH/EPDCCH is used to carry Downlink Control Information (DCI) such as scheduling decisions and power-control commands. More specifically, the DCI includes:                downlink scheduling assignments, including Physical Downlink Shared Channel (PDSCH) resource indication, transport format, hybrid-Automatic Repeat Request (ARQ) information, and control information related to spatial multiplexing (if applicable). A downlink scheduling assignment also includes a command for power control of the Physical Uplink Control Channel (PUCCH) used for transmission of hybrid-ARQ acknowledgements in response to downlink scheduling assignments;        uplink scheduling grants, including Physical Uplink Shared Channel (PUSCH) resource indication, transport format, and hybrid-ARQ-related information. An uplink scheduling grant also includes a command for power control of the PUSCH; and        power-control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.        
One PDCCH/EPDCCH carries one DCI message containing one of the groups of information listed above. As multiple terminals can be scheduled simultaneously, and each terminal can be scheduled on both downlink and uplink simultaneously, there must be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on separate PDCCH/EPDCCH resources, and consequently there are typically multiple simultaneous PDCCH/EPDCCH transmissions within each subframe in each cell. Furthermore, to support different radio channel conditions link adaptation can be used, where the code rate of the PDCCH/EPDCCH is selected by adapting the resource usage for the PDCCH/EPDCCH to match the radio channel conditions.
Carrier Aggregation (CA)
The LTE Rel-10 standard supports bandwidths larger than 20 MHz One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular, for early LTE Rel-10 deployments, it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CCs, where each CC has, or at least has the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 4. A CA-capable User Equipment (UE) is assigned a Primary Cell (PCell) which is always activated, and one or more Secondary Cells (SCells) which may be activated or deactivated dynamically.
The number of aggregated CCs as well as the bandwidth of the individual CCs may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same, whereas an asymmetric configuration refers to the case where the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal. A terminal may, for example, support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
In addition, a key feature of CA is the ability to perform cross-carrier scheduling. This mechanism allows a (E)PDCCH on one CC to schedule data transmissions on another CC by means of a 3-bit Carrier Indicator Field (CIF) inserted at the beginning of the (E) PDCCH messages. For data transmissions on a given CC, a UE expects to receive scheduling messages on the (E)PDCCH on just one CC—either the same CC, or a different CC via cross-carrier scheduling. This mapping from (E)PDCCH to PDSCH is also configured semi-statically.
Wireless Local Area Network (WLAN)
In typical deployments of a Wireless Local Area Network (WLAN), Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is used for medium access. This means that the channel is sensed to perform a Clear Channel Assessment (CCA), and a transmission is initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is deemed to be Idle. When the range of several WLAN Access Points (APs) using the same frequency overlap, this means that all transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP which is within range can be detected. Effectively, this means that if several APs are within range, they will have to share the channel in time, and the throughput for the individual APs may be severely degraded.
A general illustration of a Listen Before Talk (LBT) mechanism is shown in FIG. 5. After a Wi-Fi station A transmits a data frame to a station B, station B shall transmit the Acknowledgement (ACK) frame back to station A with a delay of 16 μs, where this duration is referred to as the Short Inter-Frame Space (SIFS). Such an ACK frame is transmitted by station B without performing a LBT operation. To prevent another station interfering with such an ACK frame transmission, a station shall defer for a duration of 34 μs (referred to as the Distributed Inter-Frame Space (DIFS)) after the channel is observed to be occupied before assessing again whether the channel is occupied.
Therefore, a station that wishes to transmit first performs a CCA by sensing the medium for a fixed duration DIFS. If the medium is idle, then the station assumes that it may take ownership of the medium and begin a frame exchange sequence. If the medium is busy, the station waits for the medium to go idle, defers for DIFS, and waits for a further random backoff period. To further prevent a station from occupying the channel continuously and thereby prevent other stations from accessing the channel, it is required for a station wishing to transmit again after a transmission is completed to perform a random backoff.
The Point Coordination Function (PCF) Inter-Frame Space (PIFS) is used to gain priority access to the medium, and is shorter than the DIFS duration. Among other cases, it can be used by stations operating under PCF, to transmit Beacon Frames with priority. At the nominal beginning of each Contention-Free Period (CFP), the Point Coordinator (PC) shall sense the medium. When the medium is determined to be idle for one PIFS period (generally 25 μs), the PC shall transmit a Beacon frame containing the Contention Free (CF) Parameter Set element and a delivery traffic indication message element.
Load-Based CCA in Europe Regulation EN 301.893
For a device not utilizing the Wi-Fi protocol, European Telecommunications Standards Institute (ETSI) “Broadband Radio Access Networks (BRAN); 5 GHz high performance RLAN; Harmonized EN Covering the essential requirement of article 3.2 of the R&TTE Directive,” EN 301.893, Version 1.7.1, June 2012 (herein referred to as “EN 301.893”), provides the following requirements and minimum behavior for the load-based clear channel assessment.                1. Before a transmission or a burst of transmissions on an operating channel, the equipment shall perform a CCA check using “energy detect.” The equipment shall observe the Operating Channel(s) for the duration of the CCA observation time which shall be not less than 20 μs. The CCA observation time used by the equipment shall be declared by the manufacturer. The Operating Channel shall be considered occupied if the energy level in the channel exceeds the threshold corresponding to the power level given in point 5 below. If the equipment finds the channel to be clear, it may transmit immediately (see point 3 below).        2. If the equipment finds an Operating Channel occupied, it shall not transmit in that channel. The equipment shall perform an extended CCA check in which the Operating Channel is observed for the duration of a random factor N multiplied by the CCA observation time. N defines the number of clear idle slots resulting in a total Idle Period that need to be observed before initiation of the transmission. The value of N shall be randomly selected in the range 1 . . . q every time an Extended CCA is required and the value stored in a counter. The value of q is selected by the manufacturer in the range 4 . . . 32. This selected value shall be declared by the manufacturer (see clause 5.3.1 q of EN 301.893, v. 1.7.1). The counter is decremented every time a CCA slot is considered to be “unoccupied.” When the counter reaches zero, the equipment may transmit.                    NOTE 1: The equipment is allowed to continue Short Control Signaling Transmissions on this channel, providing it complies with the requirements in clause 4.9.2.3 of EN 301.893, v. 1.7.1.            NOTE 2: For equipment having simultaneous transmissions on multiple (adjacent or non-adjacent) operating channels, the equipment is allowed to continue transmissions on other Operating Channels providing the CCA check did not detect any signals on those channels.                        3. The total time that an equipment makes use of an Operating Channel is the Maximum Channel Occupancy Time which shall be less than (13/32)×q ms, with q as defined in point 2 above, after which the device shall perform the Extended CCA described in point 2 above.        4. The equipment, upon correct reception of a packet which was intended for this equipment, can skip CCA and immediately (see note 3) proceed with the transmission of management and control frames (e.g. ACK and Block ACK frames). A consecutive sequence of transmissions by the equipment, without it performing a new CCA, shall not exceed the Maximum Channel Occupancy Time as defined in point 3 above.                    NOTE 3: For the purpose of multi-cast, the ACK transmissions (associated with the same data packet) of the individual devices are allowed to take place in a sequence.                        5. The energy detection threshold for the CCA shall be proportional to the maximum transmit power (PH) of the transmitter: for a 23 dBm equivalent isotropically radiated power (e.i.r.p.) transmitter, the CCA threshold level (TL) shall be equal or lower than −73 dBm/MHz at the input to the receiver (assuming a 0 dBi receive antenna). For other transmit power levels, the CCA TL shall be calculated using the formula: TL=−73 dBm/MHz+23−PH (assuming a 0 dBi receive antenna and PH specified in dBm e.i.r.p.).An example to illustrate EN 301.893 is provided in FIG. 6.Licensed Assisted Access (LAA) to Unlicensed Spectrum Using LTE        
Until now, the spectrum used by LTE is dedicated to LTE. This has the advantage that an LTE system does not need to coexist with other, non-3GPP radio access technologies in the same spectrum, and spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited and, therefore, cannot meet the ever-increasing demand for larger throughput from applications/services. Therefore, a new study item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum.
With Licensed Assisted Access (LAA) to unlicensed spectrum, as shown in FIG. 7, a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. In this application, a SCell in unlicensed spectrum is denoted as LAA Secondary Cell (LAA SCell). The LAA SCell may operate in downlink-only mode or operate with both uplink and downlink traffic. Furthermore, in future scenarios, the LTE nodes may operate in standalone mode in license-exempt channels without assistance from a licensed cell. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LAA as described above needs to consider coexistence with other systems such as a IEEE 802.11 system (i.e., a Wi-Fi system).
To coexist fairly with the Wi-Fi system, transmission on the SCell must conform to LBT protocols in order to avoid collisions and causing severe interference to on-going transmissions. This includes both performing LBT before commencing transmissions, and limiting the maximum duration of a single transmission burst. The maximum transmission burst duration is specified by country and region-specific regulations, for e.g., 4 ms in Japan and 13 ms according to EN 301.893. An example of LAA to unlicensed spectrum using LTE CA and LBT to ensure good coexistence with other unlicensed band technologies is shown in FIG. 8 with different examples for the duration of a transmission burst on the LAA SCell constrained by a maximum allowed transmission duration of 4 ms.
There is currently no LBT specification for LTE as it has, so far, operated exclusively in licensed spectrum. Reusing the existing LBT procedure for load-based equipment in EN 301.893 will lead to LAA capturing the majority of the channel access opportunities and starvation of Wi-Fi devices, due to the lack of defer periods, shorter CCA durations compared to DIFS/PIFS, and several other differences from the Wi-Fi CSMA/CA channel contention procedure. Furthermore, it is not technically feasible for LAA LTE to exactly reuse the existing Wi-Fi CSMA/CA protocol. Therefore, there is a need for LBT procedures for LTE in the context of LAA.